The present invention relates to a process and apparatus for transferring electrostatic latent images, and more particularly, for transferring electrostatic latent images from an electrostatic receptor onto a copy sheet.
The process of transferring an electrostatic latent image includes the step of first forming an image on the surface on an electrostatic receptor by selectively discharging the surface of the receptor by a Corjet head or any other suitable ion projection device. The latent electrostatic image on the electrostatic receptor is then suitably developed with a toner material and then transferred to a copy sheet by bringing the copy sheet into intimate contact with the developed image on the receptor.
A large electrical energy gap is generally provided between the receptor surface and the copy sheet to attract the toner onto the copy sheet. However, this electrical energy differential will often cause a premature transfer of the toner forming the toner image to the copy sheet paper before the copy sheet paper is in intimate contact with the receptor surface resulting in distortions in the transferred image such as a "halo" effect or any other imperfection.
In an effort to eliminate such drawbacks, it has been described in U.S. Pat. No. 3,147,679 to provide a first roller with a voltage of the same polarity as the charge on the image bearing surface. Subsequently, the latent image is transferred by a second roller applied with a high voltage of polarity opposite to that of the charge of the latent image. A potential of polarity opposite to that of the latent image on the copy sheet is placed on the dielectric layer of the copy sheet in order to prevent abnormal discharge between the photosensitive surface and the copy sheet immediately before contact is achieved by the first roller. The variable potential source further applies to a separating roller a voltage of the same polarity as the first roller to prevent damage due to discharge when the copy paper is separated. This is rather a complex and costly system.
An improvement is disclosed in U.S. Pat. No. 4,056,390 in which a copy sheet is brought into contact with a photosensitive surface by a first roller having an electrical resistance value substantially higher than the face resistance value of the copy sheet. A photosensitive surface is driven by a grounded roller and an electrically conductive lining element of the copy sheet is substantially grounded by an electrically conductive second roller to transfer the electrostatic latent image by air breakdown discharge under the influence of the first and second rollers. Other biasing paper feed devices at the transfer station are disclosed in U.S. Pat. Nos. 4,396,273 and 4,415,254. The difficulty with these systems, however, is that they are also relatively complex and are directed to electrophotographic apparatus and not to electrostatic receptors having a relatively high background voltage.
It is an object of the present invention to provide a new and improved transfer means that is simple and reliable. It is another object of the present invention to provide a pretransfer baffle in an electrostatic printing system that prevents image degradation and in particular a halo effect in the transfer of toner from a high electrical charge image receptor to a copy sheet.
Further advantages of the present invention will become apparent as the following description proceeds and the features characterizing the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.